Devices of this type are generally known as turbulence agitators or "turbulators", and comprise a strip of material twisted into a helix which is placed longitudinally in the tube so as to cause a turbulent flow along it. Such a device is described in French patent Specification No. 73.33390, published under U.S. Pat. No. 2,244,149. As known, this type of agitator has at least one helical thread.
The main disadvantage of these agitators lies in the fact that they cause a considerable pressure drop in the tube in which they are placed, this pressure drop being the same in whatever direction the fluid is flowing, in relation to the agitator.
This equal pressure drop is explained by the fact that the agitator presents similar surfaces to oppose the flow of fluid in both directions. In particular, if a helical agitator such as that described in the above-mentioned specification is cut through a plane passing through the axis of the helix, a section is obtained comprising an elongate central portion from which equally-spaced and equal-sized teeth extend at certain intervals, the profile of each tooth being symmetrical with respect to a line which bisects it and is perpendicular to the axis of the helix.